Method of preparing fibrous magnesium oxide

ABSTRACT

This specification discloses a method of preparing fibers of magnesium oxide, nickel oxide, and solid solutions thereof by forming a liquid film of reaction mixture containing a magnesium or nickel salt and as auxiliary salt; reacting said film mixture with water vapor to initiate fibrous growth at a temperature of from about 700*C to 1,200*C; continually supplying a film mixture over the fibers to sustain growth; and separating the fibers from any residual mixture, thereby to produce said fibers which are characterized by unusually high tensile strength and resistance to high temperatures.

United States Patent Bruba ker METHOD OF PREPARING FIBROUS MAGNESIUM OXIDE [75] Inventor: Burton D. Brubaker, Midland,

Mich.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: April 13, 1970 [2]] Appl. No.: 28,103

Related US. Application Data [63] Continuation-in-part of Ser. No. 622,046, March [0,

I967, abandoned.

[52] [1.8. CI. ..423/638, 423/636, 423/639 [51] lnt.Cl. ..COIf 5/06, C0lf5/l0, C0lf5/l2 [58] Field ofSearch ,.23/l83,20l; l06/55,58

[56] References Cited UNITED STATES PATENTS 2,l9l,56l 2/l940 Farnsworth ..23/20l X 1,325,932 l2/l9l9 Dutt ..23/201 Jan. 16, 1973 FOREIGN PATENTS OR APPLICATIONS 497,l l9 l2/l938 Great Britain ..23/30l OTHER PUBLICATIONS Pavlyuchenko et al.; Chemical Abstract" Vol. 64, I966, Col. l2,l83(b) [57] ABSTRACT This specification discloses a method of preparing fibers of magnesium oxide, nickel oxide, and solid solutions thereof by forming a liquid film of reaction mixture containing a magnesium or nickel salt and as auxiliary salt; reacting said film mixture with water vapor to initiate fibrous growth at a temperature of from about 700C to l,200C; continually supplying a film mixture over the fibers to sustain growth; and separating the fibers from any residual mixture, thereby to produce said fibers which are characterized by unusually high tensile strength and resistance to high temperatures.

6 Claims, No Drawings METHOD OF PREPARING FIBROUS MAGNESIUM OXIDE This application is a continuation-in-part of Ser. No. 622,046, filed Mar. 10, 1967 now abandoned.

A primary object of the present invention is to provide a new and novel method of preparing fibers or socalled whiskers of magnesium oxide, nickel oxide and/or solid solutions thereof, which is relatively simple to carry out, lower in cost than the prior art methods of making such fibers, and which has the capability of producing large yields of the fibrous product.

The above and other objects and advantages are ob tained in accordance with the present invention in a method comprising: (a) forming a liquid film of a mixture of a metal salt with at least one auxiliary salt, the proportion of said metal salt being within the range of from about 2 to about 90 mole percent of the mixture; (b) reacting said mixture with water vapor at a temperature within a range of from about 700C to about l,200C and preferably from about 800C to about l,lC to initiate metal oxide fiber growth; (c) continually supplying a liquid film of said mixture over the fibers to sustain growth; and (d) separating the fibrous metal oxide product from any residual mixture, for example, by treating it with an aqueous leaching solution. The fibrous oxide product so-prepared and obtained comprises, after drying, a loosely packed, bulky, fibrous mass of magnesium oxide, nickel oxide, or solid solutions thereof having a low density, the individual fibers having high tensile strength and high temperature resistant properties.

The term auxiliary salt as used herein means the alkali metal chlorides and bromides, alkali metal sulfates, calcium, barium or strontium chlorides, or mixtures thereof, employed in a reaction mixture with particular magnesium or nickel inorganic salts as hereinafter specified.

The term metal salt" means the chlorides, bromides and sulfates of magnesium, nickel or mixtures thereof.

ln carrying out the process of the present invention a metal salt is admixed with at least one auxiliary salt to form the reaction mixture. A double salt such as, e.g., camallite (MgCl 'KCl6H O) may also be conveniently used. In such instance, no separate addition of auxiliary salt is necessary. A mixture of magnesium or nickel chloride together with either a sodium, lithium or potassium halide salt or calcium chloride is preferred as the reaction mixture composition. Various other combinations of e.g., the magnesium salt and auxiliary salts may also be employed in the composition. When solid solutions of magnesium oxide and nickel oxide fibers are desired, the composition will comprise a mixture of magnesium or nickel salts in amounts within those specified herein depending on the solid solution fiber composition desired.

When magnesium chloride is employed as the meta salt with alkali metal chlorides or bromides, such as, for example, the chlorides or bromides of sodium, lithium or potassium, the magnesium chloride is normally employed in an amount of from about 2 to about 90 mole percent, i.e. from 2 to 90 moles per hundred moles of reaction mixture. When magnesium chloride is used in the reaction mixture with calcium, barium or strontium chloride auxiliary salt, normally from 2 to 40 mole percent of the magnesium chloride is employed. This 40 percent limitation can be increased as and if sodium,

lithium, or potassium halide auxiliary salts are added to said calcium, barium, or strontium salts in increasing amounts.

When magnesium bromide is used together with, e.g. sodium, lithium, or potassium chloride or bromide salts, from 5 to mole percent of the magnesium salt is normally used. However, the calcium, barium or strontium halide auxiliary salts are substantially inoperable in the reaction composition with magnesium bromide.

Magnesium sulfate is normally employed in an amount of from about 2 to about 50 mole percent in combination with sodium, lithium, or potassium chloride or bromide auxiliary salts in the reaction composition. With calcium, barium, or strontium halide chloride auxiliary salts, the magnesium sulfate is used in an amount of from about 2 to about 30 mole percent.

In preparing the metal oxide fibers a liquid film of the mixture is formed. This is accomplished, for example, by pouring the mixture in molten form on a flat or inclined substrate, or, since the mixture is highly wettable, by allowing the liquid to flow up the side of a reaction vessel, e.g. a crucible. To initiate fibrous metal oxide growth, the film mixture is reacted with water vapor, e.g. in the atmosphere surrounding the film, at a temperature within the range from about 700C to l,200C, preferably 800C to l,l00C. The time of reaction at the temperature specified preferably should be maintained for at least about 30 minutes, and more preferably for about 2 hours to about 5 hours. Heating times greater than about 5 hours do not significantly increase the fiber length over that obtained during the 2 to 5 hours heating time. It is, however, not particularly detrimental if greater times are employed.

During reaction, metal oxide is formed by the reaction of the metal ion (M**) with a hydroxyl ion (OH) and an anion (X'):

The hydroxyl ions are formed by reaction of the water vapor with an anion (X') Where the reaction mixture is a thick mass, the metal oxide forms as granular crystals. However, when the reaction takes place in a film of reaction mixture, as specified in the present method, the growth is unidirectional, causing metal oxide fiber to form. This is due in part to the selective reaction of the hydroxyl ions with the closest edge of the metal oxide nuclei. The top of the nucleus, e.g. the edge nearest the film surface, reacts with the hydroxyl ions more readily than the sides" of the nucleus which are shielded by the mixture film.

To sustain the fibrous growth a liquid film of mixture is continually supplied over the fibers, e.g. by continually pouring molten mixture over the fibers to maintain a film or allowing capillary action to crawl or siphon" a liquid film over the growing fibers.

When the fibers reach the desired length, e.g. a few microns to l0,000-l2,000 microns, the fibers are separated from any residual mixture. The fibers are contacted with an aqueous solution to leach out the soluble mixture leaving behind the metal oxide fibrous product, which is thereafter normally dried either in an oven at moderate temperatures such as,'e.g. 100C or air dried at ambient temperatures.

As an alternative to leaching out the auxiliary salts, where little or no unreacted magnesium salts are present, the fibers can be further heated to vaporize off the auxiliary salts leaving the metal oxide fibers as product.

The fibrous oxide yield from the present novel process varies depending on the reaction time, temperature, atmosphere, and reaction composition. In general, the fiber yields can vary from, e.g. about 20 to about 75 percent, based on a 100 percent conversion of the magnesium or nickel salt in the reaction mixture to the fibrous oxide.

The following examples serve to illustrate the present novel process.

Example I A sample of natural carnallite salt from Cardona, Suria, Spain weighing about 40 grams was placed in a Vycor brand crucible (crucible comprising about 96 percent silica). Carnallite (MgCl 'KCl-6H O) is an example of a naturally occurring double salt which does not require the separate addition of an auxiliary salt. The crucible and contents were placed in a furnace preheated to about 900C. The crucible was actually maintained at the 900 C temperature for about 1.5 hours. The furnace had openings so that air circulation could take place and gaseous byproducts could be exhausted. The dew point of the air so-circulated was about 2C.

Upon heating, the carnallite became molten and crawled upthe side and over the top of the crucible, forming a film on the inner and outer sides of the crucible. The mixture reacted with the water vapor in the atmosphere to form magnesium oxide fibers on the inner and outer sides of the crucible, with the maximum deposit being at the rim. A film of liquid mixture from the reservoir in the crucible was continually supplied over the fibers by capillary action. The fibers were cooled and leached in water to remove any residual carnallite. The magnesium oxide fibers so-deposited were from about 100 microns to about 2,500 microns in length. The majority of fibers were about 2,000 microns in length. A 42 percent conversion or yield was obtained.

Example ll A number of binary reaction compositions were prepared using magnesium chloride (or its hexahydrate) and various alkali metal and alkaline earth metal auxiliary inert salts. The composition of these binaries is shown in Table l below. Each binary reaction composition was placed in a Vycor crucible and the crucible heated in an oven to temperatures and for times as indicated in Table I. An ambient air atmosphere was maintained over the reaction as it proceeded, having a sufficient dew point for growth of fibers in accordance with the present invention. The mechanisms for the formation of the liquid film of mixture, the reaction of 'the'mixture with water vapor, the continued supplying of film, and the separation of fibers were the same as Example I. The magnesium oxide fiber lengths noted are a reasonable average of optical microscopy.

TABLE I a. (MgCl, salt potassium chloride auxiliary salt) Mole Heating Time Temp Fiber Length 5 MgCl, Hours C in Microns 9O 2 900 60 75 2 900 40 6O 2 900 3500 50 Z 900 I300 40 2 900 2000 25 2 900 700 10 WM .9. ME 40 2 2 .900. 500

b. (MgCl salt+ sodium chloride auxiliary salt) 3 900 700 in 00 i 400 c. (MgCl salt-l-lithium chloride auxiliary salt) 5 3 900 I000 10 3 900 400 d. (MgCl, salt cesium chloride auxiliary salt) 68 2.5 900 300 e. (MgCl, salt KBr auxiliary salt) 50 2 900 300 f. (Mgcl salt calcium chloride auxiliary salt) 30 3 900 80 20 3 900 200 l0 3 900 400 5 3 900 500 g. (MgCl salt potassium chloride and other auxiliary salt) Mole percent Time Temp Fiber MgCl, KCl Other Hours C Length Yield 54.7 36.489 CaCl, 18 900 I50 50 30 CaCl= 20 900 I00 75 30 12.7 35.851 LiCl 2 800 300 30 6.l 38.5 55.4 LiCl 2.75 700 300 Example ill Similarly as in Example ll above various binary reaction compositions were prepared and heated for the periods and at the temperatures indicated to produce fibers of magnesium oxide. These compositions contained various magnesium salts (Mg) and various auxiliary salts (A.S.). The results are recorded in Table II below indicated proportions or reactants, heating time in hours, temperature in C, and fiber length in microns, relative to each composition.

About 19 pounds of carnallite magnesium salt, and about 5 pounds of calcium chloride were heated in a fused quartz crucible for 4 hours at 850C, then cooled. The mechanisms for the formation of the liquid film of mixture, the reaction, the continued supplying of film and the separation of fibers were the same as Example I. A fibrous magnesium oxide product was obtained wherein fibers obtained a length of about one-half an inch.

Example V The present invention may be modified or changed without departing from the spirit, scope, or substance of the present invention and it is understood that the invention is only limited as defined in the appended claims.

What is claimed is: l. A method for preparing magnesium oxide fibers Example Vl which comprises:

a. providing a mixture selected from a member of the group consisting of About 40 grams of carnallite magnesium salt and 20 grams of CaCl were heated as in Example IV for about 2.5 hours at 800C. A fibrous magnesium oxide 5 product was obtained wherein the maximum fiber length was greater than about 4,000 microns.

Various reaction mixtures were prepared containing 10 nickel chloride (NiCl2'6H2O) magnesium chloride from about 2 to about 90 mole percent magnesium (Mgcm' and auxlllry salts potass'um chloflde chloride with the balance at least one auxiliary salt (KCI) and calclum chlonde (Caclz), m a selected from the group consisting of the chlorides crucible for the time (hours), and at the Centigrade and bromides ofsodiumlithium and potassium, temperature (C) indicated in Table III in a gaseous atfrom about 2 to about 40 o percent magnesium mosphere havmg f f (cc) of at chloride and the balance at least one auxiliary salt Upon heating, the I1qu1d mixture wet the cruc ble walls selectedyfi-om the group consisting f the cmorides forming a film. Reaction of the mixture with water ot'calciumbarium and strontium, vapor caused formation of solid solution fibers. Liquid f about 5 to about 75 mole percent magnesium mixture was Supplied capillary action- Magnesium bromide and the balance at least one auxiliary salt Oxide-nickel Oxide Solid solutlo" fibers of Varying selected from the group consisting of chlorides and length were obtained. The reaction condition, and proid f Sodium, li hi d potassium, portions of materials are presented in said Table III. f m about 2 to b t 50 l r ent ma nesium Fiber composition Mole ratio Mole percent ot in mole percent in starting Time, Temp., Fiber composition Sample No. NiClz'GHzO MgCl KCI CaClz hours C. NiO MgO length of Ni-to-Mg Example Vll 7 v i sulfate and the balance at least one auxiliary salt selected from the group consisting of the chlorides and bromides of sodium, lithium and potassium, from about 2 to about mole percent magnesium sulfate and the balance at least one auxiliary salt Nickel oxide fibers in substantial length were prepared in accordance with the present invention by heating mixtures of NiCl '6l'l O, CaCl, and KCl in varying proportions for periods from 2 to 3 hours at teml t d f th fth M peratures from 800C to l,000C. The fiber lengths oba; g fg f fgz f fgfif mg e c es tained and the reaction conditions are presented in forming anucleus of magnesium oxide on a solid Table Iv below: substrate wetted by a molten mixture of step (a) by TABLE IV reacting the mixture of step (a) with water vapor at a temperature within the range of from about Sample Mole Per Cent of time temp 700C. t0 1,200C.;

c. forming a molten film with the mixture of step (a) I NiCl,-6H,O CaCl, KCI hrs. "C Length I over the nucleus; 2 5 0 95 3 400 d. reacting a portion of the film over the nucleus with 10 0 90 3 900 500 water vapor at a temperature within the range of 3 7 7 from about 700C. to l,200C. to thereby produce 4 0 3 900 350 a fiber by unidirectional growth of the magnesium o 0 0 3 900 300 Oxide reaction product; 5 e. continually supplying for at least about 30 minutes 6 0 90 2 800 50 the liquid film over the fiber ends to sustain only 16 15 69 2 1000 700 unidirectional growth of the magnesium oxide; 7 20 47 i 33 2 I000 250 f. separating the magnesium oxide fibers from any v 7 7 res1dualm1xture;and Other combinations of magnesium or nickel salts or gvering a fibrous mass of magnesium oxide. mixtures thereof with one or more auxiliary salts in ac- 2 Th th d f l i 1 h i h reaction i cordance with the present invention can be processed i d t t a t erat r range of f m about 800C to to provide nickel oxide or magnesium oxide fibers of 5 b r1,100(j substantial length characterized by high tensile 3, The method of claim 1 wherein the molten film of strengths and resistance to high temperatures similarly step (c) is formed by pouring the molten mixture of as hereinbefore specified. step (b) onto an inclined substrate.

4. The method of claim 1 where the liquid film'of tains magnesium chloride. step (c) is formed by continually supplying the molten 6. The method of claim 1 wherein the mixture conmixture of step (b) to the fiber ends by capillary action. tains magnesium sulfate.

5. The method of claim 1 wherein the mixture con- 1 n! 4- 

2. The method of claim 1 wherein the reaction is carried out at a temperature range of from about 800*C to about 1,100*C.
 3. The method of claim 1 wherein the molten film of step (c) is formed by pouring the molten mixture of step (b) onto an inclined substrate.
 4. The method of claim 1 where the liquid film of step (c) is formed by continually supplying the molten mixture of step (b) to the fiber ends by capillary action.
 5. The method of claim 1 wherein the mixture contains magnesium chloride.
 6. The method of claim 1 wherein the mixture contains magnesium sulfate. 